Lung cancer has been the most common cancer in the world since 1985, with approximately 163,510 deaths expected to occur in 2005 in the United States. Even with current treatments involving surgery, chemotherapy, and/or radiotherapy, overall 5-year survival in the United States is still only approximately 15 percent; in developing countries, 5-year survival rates are about 9 percent (Parkin et al., CA Cancer J. Clin. (2005) 55:74-108). The overwhelming majority of lung cancers in both sexes can be attributed to smoking. However, approximately 10% of lung cancers arise in individuals who smoked less than 100 cigarettes in a lifetime (“never smokers”).
Several proto-oncogenes encoding components of the ERBB/HER signaling pathway are known to be mutated in lung cancers, almost exclusively in adenocarcinomas. The most common mutations affect the KRAS and EGFR genes. Approximately 15 to 30% of adenocarcinomas have KRAS mutations (Rodenhuis and Slebos, Am. Rev. Respir. Dis. (1990) 142:S27-30; Suzuki et al., Oncogene (1990) 5:1037-1043), while EGFR mutations are found in about 10% of adenocarcinomas in the United States and in much higher percentages of patients in East Asia (Pao et al., PLoS Med (2005) 2:e17). EGFR and KRAS mutations are mutually exclusive, suggesting that they have functionally equivalent or overlapping roles in lung tumorigenesis.
The majority of mutations in KRAS in human lung adenocarcinomas are found in exon 2, leading to missense amino acid substitutions in codons 12 or 13 (Rodenhuis et al., Cancer Res. (1988) 48:5738-5741). Nearly 90% of EGFR mutations identified in human lung cancer occur in two “hotspots”. Half of these mutations are various multi-nucleotide in-frame deletions that eliminate a highly conserved four amino acid sequence (LREA) encoded in exon 19. The other “hotspot” mutations are point mutations in exon 21 that result in a specific amino acid substitution (i.e., lysine to arginine) at position 858 (L858R). The remaining 10-15% are nucleotide substitutions, deletions, or insertions/duplications found in exons 18-21, outside of the common sites of mutation (Lynch et al., N Engl. J. Med. (2004) 350:2129-2139; Paez et al., Science (2004) 304:1497-1500; Pao et al., Proc. Natl. Acad. Sci. USA (2004) 101:13306-13311).
Most of the somatic mutations in the EGFR described above are associated with sensitivity of human lung tumors to the tyrosine kinase inhibitors gefitinib (IRESSA™, Astra-Zeneca) and erlotinib (TARCEVA™, OSI Pharmaceuticals, Genentech). Although these two small molecules were developed as inhibitors of the wild type EGFR tyrosine kinase, EGFR with the common mutations in the kinase domain confer an augmented sensitivity to these drugs. To date, gefitinib and erlotinib are the only two targeted agents clinically available for treatment of human non-small cell lung cancer (adenocarcinoma, squamous cell carcinoma and large cell carcinoma).
In most patients who initially respond to crlotinib and gcfitinib, the cancer resumes detectable growth within 6 months to two years. This loss of sensitivity to gefitinib and erlotinib arises co-incidentally with a secondary mutation in exon 20 of the EGFR, which leads to substitution of methionine for threonine at position 790 (T790M) in the kinase domain, in addition to the primary drug-sensitive mutation. Activating EGFR mutations (e.g. L858R) seem to occur preferentially in cis with the T790M mutation. This T790M mutation has also recently been identified in a family with a history of adenocarcinoma (Bell et al., Nat. Genet. (2005) 37:1315-1316). Biochemical analyses of transfected cells and growth inhibition studies with lung cancer cell lines demonstrated that the T790M mutation confers resistance to the EGFR mutants that were usually sensitive to either gefitinib or erlotinib (Kobayashi et al., N. Eng. J. Med. (2005) 352:786-792).
Tumors arising from mutant EGFR require the continuous expression/activity of the oncogene for survival. This is based on the regression of mutant EGFR lung tumors that were treated with erlotinib and gefitinb, and has also been demonstrated for lung tumors arising in mice with controlled expression of oncogenic egfr (Politi et al., Genes Dev. (2006) 20:1496-1510) and kras (Fisher et al., Genes Dev. (2001) 15:3249-3262). The mutant oncogene may alter a normal cell so that continued expression or function of the oncogene is required to prevent the cell from entering an apoptotic (or differentiation) pathway. Very likely, the protein encoded by the mutant oncogene activates multiple signaling pathways in such a way that sudden loss of a signal dependent on a mutant oncoprotein removes a block to other signals that direct apoptosis or differentiation.
There are currently no approved therapeutic drugs that are effective against lung tumors with the T790M-EGFR mutation. Moreover, there has been no success in developing agents that would target KRAS. Given the high prevalence of KRAS mutations in lung and other cancers, a drug that targets this type of cancer is urgently needed.